Nitrate Transport and Retention in Western European Catchments Are Shaped by Hydroclimate and Subsurface Properties

Abstract

Nitrogen (N) can be a limiting nutrient in terrestrial, freshwater, and marine ecosystems (Webster et al., 2003). However, the N cycling in these ecosystems is modified and disturbed by humans through inputs from atmospheric deposition, agricultural fertilizers and wastewater. High N inputs especially in economically developed countries have led to increased riverine nitrogen fluxes, causing ecological degradation in aquatic systems and posing a threat to drinking water safety (Dupas et al., 2016;Sebilo et al., 2013;Wassenaar, 1995). Diffuse agricultural sources (mineral fertilizer and manure) constitute most of the N emissions into waters in European countries (Bouraoui & Grizzetti, 2011;Dupas et al., 2013).

Characterizing Catchment‐Scale Nitrogen Legacies and Constraining Their Uncertainties

“…(2012) found a value of the mean travel time in the groundwater of 25 years Ehrhardt et al. (2021) established that the overall mode travel time for total flow are in the range 0–20 years for different subcatchments of the WRB and in particular in the range 0–10 years for most subcatchments. Therefore previous studies cover a large spectrum of travel time values, which includes our estimates.…”

“…Some past N modeling studies covering a large range of catchments across Germany and France have not accounted for N point sources (Dupas et al., 2020; Ehrhardt et al., 2021). Based on our simulation results, we recommend the consideration of N point sources and their temporal variations in future N modeling analyses over the WRB.…”

“…We perform Monte Carlo simulations for each of the eight subcatchments and we sequentially apply the seven soft rules, thus progressively reducing the number of behavioral parameter sets. Following previous studies (Dupas et al., 2020; Ehrhardt et al., 2021; Van Meter et al., 2017), we use the entire time series of in‐stream N observations to identify behavioral simulations. We note that the goal of our analysis here is not to predict future catchment N export but to analyze the uncertainty in the simulations and the value of different types of data to constrain the simulations.…”

“…Koeniger et al (2008) reports values of the mean groundwater travel time in the range 8-93 years (37 years for total flow) for Hemelingen, based on long-term tritium isotope data in combination with simulations from a hydrologic model. Hirt et al (2012) found a value of the mean travel time in the groundwater of 25 years Ehrhardt et al (2021) established that the overall mode travel time for total flow are in the range 0-20 years for different subcatchments of the WRB and in particular in the range 0-10 years for most subcatchments. Therefore previous studies cover a large spectrum of travel time values, which includes our estimates.…”

“…In this respect, some past modeling studies investigate the time lag between the trajectory of N inputs to the terrestrial system and the in-stream N levels using empirical approaches, for example, the studies of Chen et al (2014), Dupas et al (2020), Ehrhardt et al (2021) and (further details are reported in the review of Chen et al, 2018). Yet, such modeling approaches are typically based on lumped transfer functions that relate the N inputs to the N stream export at catchment scale and they do not explicitly disentangle the role of biogeochemical and hydrologic legacies.…”